Problems with Samba 3.x domain

Hi,
I am running Linux Fedora core 2. In my smb.conf, firstly, use smbpasswd as
password backend. I set roving (roaming) profiles, I create machine accounts
and user accounts in passwd and in smbpasswd, I net groupmap Nt groups to
Unix's groups, and everything works fine. I can join Xp clients to domain, I
can log on with users defined, I got home drives mapped as defined in
smb.conf, and so on.

Then, as I read that it is most secure to store nt password in tdbsam (and
as I am not using AD, and I have less then 250 clients in my domain), i
decide to make the change. So, I simply change the line in smb.conf, form
"passdb backend =smbpasswd" to "passdb backend=tdbsam". I read dozens of
pages, howto's and man's, and it seems you have nothing else to do. But,
from this moment on, things stop working. I simply can't log on the domain
no more, getting the message "Domain controller unavailable....".

What's wrong ??? If necessary, I will paste in the lines of my smb.conf.

Please, give me some hints, or some links where to read interesting facts on
Samba (I read the pdf's of "Samba howto's collection", "Using Samba" from
O'Reilly, docs from the official samba web site....but I can't find answers.

Thanks to all.